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Date: Wed, 12 Dec 2012 18:24:31 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc: tglx@...utronix.de, peterz@...radead.org,
paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org,
vincent.guittot@...aro.org, tj@...nel.org, sbw@....edu,
amit.kucheria@...aro.org, rostedt@...dmis.org, rjw@...k.pl,
wangyun@...ux.vnet.ibm.com, xiaoguangrong@...ux.vnet.ibm.com,
nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU
offline from atomic context
On 12/12, Oleg Nesterov wrote:
>
> On 12/11, Srivatsa S. Bhat wrote:
> >
> > IOW, the hotplug readers just increment/decrement their per-cpu refcounts
> > when no writer is active.
>
> plus cli/sti ;) and increment/decrement are atomic.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
OOPS, sorry I was going to say "adds mb()".
And when I look at get_online_cpus_atomic() again it uses rmb(). This
doesn't look correct, we need the full barrier between this_cpu_inc()
and writer_active().
At the same time reader_nested_percpu() can be checked before mb().
Oleg.
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